Basically anything that it would make sense for Moat to block, is my impression. So for example, Ill Gotten Gains could be an attack, but couldn't be blocked since it doesn't attack when played. On the flip side, Council Room could be an attack but it wouldn't usually make sense to block it.

I think it's pretty obvious to me - it's cards with an on-play effect that are usually detrimental to an opponent.

Right - an on-play effect that looks like it is clearly trying to hurt opponents.

Ill-Gotten Gains would have been an attack if attacks and Moats were worded so as to handle that from the start. Things like Council Room or Embassy's when-gain would never have been attacks, even though you can argue that sometimes they hurt you.

I think it's pretty obvious to me - it's cards with an on-play effect that are usually detrimental to an opponent.

Right - an on-play effect that looks like it is clearly trying to hurt opponents.

Ill-Gotten Gains would have been an attack if attacks and Moats were worded so as to handle that from the start. Things like Council Room or Embassy's when-gain would never have been attacks, even though you can argue that sometimes they hurt you.

I would argue that Embassy's when-gain hurts more often than some actual Attacks in the game.

I think it's pretty obvious to me - it's cards with an on-play effect that are usually detrimental to an opponent.

Right - an on-play effect that looks like it is clearly trying to hurt opponents.

Ill-Gotten Gains would have been an attack if attacks and Moats were worded so as to handle that from the start. Things like Council Room or Embassy's when-gain would never have been attacks, even though you can argue that sometimes they hurt you.

I would argue that Embassy's when-gain hurts more often than some actual Attacks in the game.

I think it's pretty obvious to me - it's cards with an on-play effect that are usually detrimental to an opponent.

Right - an on-play effect that looks like it is clearly trying to hurt opponents.

Ill-Gotten Gains would have been an attack if attacks and Moats were worded so as to handle that from the start. Things like Council Room or Embassy's when-gain would never have been attacks, even though you can argue that sometimes they hurt you.

I would argue that Embassy's when-gain hurts more often than some actual Attacks in the game.

And yet, if it were possible to make it an attack, it still wouldn't be one.

Simplicity. There's no reason in adding "may" on Council room when 99% of the time you're going to want that card.

Correct, and it's more simplicity than it may look like. An option that almost always goes one way needs a good reason to be an option (in Dominion a main source of this is getting to drop keeping-you-honest stuff, e.g. "you may" on new Moneylenders).

Basically anything that it would make sense for Moat to block, is my impression. So for example, Ill Gotten Gains could be an attack, but couldn't be blocked since it doesn't attack when played. On the flip side, Council Room could be an attack but it wouldn't usually make sense to block it.

Also, maybe council room should be an attack because lighthouse would stop the draw. That would be interesting

As a bit of a thought experiment, how different would things be if there were an "Attack" keyword on cards, rather than a type? The keyword would mark something that effects other players, and it's something you can react to regardless of whether it happens as part of playing the card or otherwise.

For example, Witch would become "+2 Cards. Attack: Each other player gains a Curse.," Minion would become "Choose one: +$2, or discard your hand, +4 Cards, and Attack: ...", but you could also have Ill-Gotten Gains: "+$1 ... When you gain this, Attack: Each other player gains a Curse."

Similarly, things like Moat become "+2 Cards. When another player uses an Attack, you may reveal this ..." and Squire becomes "... When you trash this, gain a card with an Attack".

Some of the immediate effects I can think of:

Attacks now work a little closer to new players' intuition of them.

Optional attacks (like Minion and Pirate Ship) favour the other players more, since the person playing the card has to choose whether to attack before it gets reacted to.

Still not obvious what happens with Masquerade or Possession (or Embassy, I guess), and regardless of how you treat them someone would probably complain.

Introduces a new keyword in card text, which is something mostly avoided in the game (at least up until Renaissance).

Probably changes the design space in a way I haven't anticipated.

For the record, I think the current solution is a good one, I'm just looking at an alternative one and wondering how it would affect things.

For example, Witch would become "+2 Cards. Attack: Each other player gains a Curse.," Minion would become "Choose one: +$2, or discard your hand, +4 Cards, and Attack: ...", but you could also have Ill-Gotten Gains: "+$1 ... When you gain this, Attack: Each other player gains a Curse."

I have worked out this exact approach. It probably would have been better. Something to remember for future games.